This disclosure relates to planar gas sensors, and, more particularly, to heater patterns for planar gas sensors that yield a reduction in the incidence of cracking attributable to tensile stresses at the edges of the planar gas sensors.
Gas sensors, and in particular oxygen sensors, are used in combustion engines to control the air/fuel ratio in the combustion chamber so that the air/fuel ratio remains at or near its proper stoichiometric value. Maintaining the proper stoichiometric value allows for the improvement of fuel consumption and the minimization of impurities in an exhaust gas. An oxygen sensor typically includes an oxygen sensing element having an ion-conductive solid electrolytic plate on which porous electrodes are disposed. A difference in potential corresponding to the difference in oxygen content between the gas and the reference air is generated by the oxygen sensing element, is quantified, and is used to adjust the air/fuel ratio in the combustion chamber.
The proper functioning of the oxygen sensing element is typically dependent upon its temperature. Because a significant amount of time is often required for the oxygen sensor to become active after startup of the engine, the air/fuel ratio is difficult to control during that time. Heaters are, therefore, oftentimes incorporated into the oxygen sensing system to more quickly bring the oxygen sensing elements up to a temperature at which the most efficiency can be realized.
Typical heaters in planar sensors are formed in various patterns on one face of the oxygen sensing element. Irregularities in the patterning of the heater trace can give rise to xe2x80x9chotspotsxe2x80x9d. These hotspots are the primary locations for failure of the heater because of opening of the heater trace. Such a design attempts to create a uniform temperature profile across the sensor element by adjusting the heat input through patterning of a single heater trace. Heater patterns such as these are difficult to control because the balance of the heat input between the center and the edges of the pattern changes as the temperature changes. Variations in the heating profile oftentimes cause xe2x80x9chotspotsxe2x80x9d within the oxygen sensing element, which result in thermal shock. In such a configuration, because the oxygen sensing element is usually fabricated from a ceramic material, differing rates of expansion often cause tensile stresses to be experienced along the interfaces of the hotter and colder areas. Such tensile stresses may, over time, cause the oxygen sensing elements to fracture and function improperly, thereby communicating inaccurate information for the control of the air/fuel ratio. In such an instance, the oxygen sensor will require replacement to ensure maximum efficiency of the system operation.
A heater pattern for a heater of a gas sensor in which a temperature profile is manipulated through the use of separate thermistor elements to reduce the number of hotspots therein is described below. The heater pattern is defined by first and second thermistor elements in communication with each other in an electrically parallel configuration. In a first embodiment, the first thermistor element extends substantially about a perimeter of the substrate and is typically formed of a material having a specific thermal coefficient of resistivity. The second thermistor element extends across a portion of the substrate intermediate the perimeter of the substrate and is formed of a material having a thermal coefficient of resistivity that is higher than the thermal coefficient of resistivity of the first thermistor element. In a second embodiment, in order to further reduce the incidence of hotspots in a heater, each thermistor element is formed of first and second conductors disposed in a spaced relationship and in communication with each other through a cross conductor. The thermistor elements are screen printed onto a substrate to a thickness of about 5 microns to about 50 microns. A method of heating the gas sensor with the heater pattern includes disposing the two thermistor elements in an electrically parallel configuration over a surface of the substrate and passing an electric current through the elements.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.